1. Field of the Invention
The invention relates to a receiver having a tuning system in which a tuning oscillator is synchronized with a stepped-frequency signal having a frequency which can be varied in steps. The invention also relates to the tuning system as such, and to a method of tuning.
2. Background Art
U.S. Pat. No. 5,150,078 describes a prior-art frequency synthesizer for Doppler radar and communication receivers. The prior-art frequency synthesizer comprises two phase-locked loops (PLLs). The first PLL is a fine or VHF step-tuning loop which provides a fine frequency-step signal. The second or L-band PLL converts the fine frequency-step signal into an L-band frequency signal. It includes an L-band voltage- controlled oscillator (VCO) whose output signal is divided by two and then mixed with the third harmonic of a reference-frequency signal to generate an offset-frequency signal. The phase of the offset-frequency signal is compared with that of a frequency-divided version of the fine frequency-step signal. Accordingly, a phase-difference signal is obtained which controls the frequency and phase of the L-band VCO.
U.S. Pat. No. 5,150,078 gives the following example. The fine frequency-step signal has a frequency of 280 MHz. Its frequency is divided by five to obtain a 56 MHz signal. The L-band VCO provides a 1388 MHz output signal. Since this output signal is divided by two, a 694 MHz signal is obtained. The reference-frequency signal is at 250 MHz and, consequently, its third harmonic is at 750 MHz. Mixing the third harmonic with the 694 MHz signal provides sum and difference frequencies of which 750xe2x88x92694=56 MHz is selected for phase comparison.
An object of the invention is to provide a receiver which, with respect to the background art, allows a better performance in terms of interference-immunity.
This object is achieved in a tuning system comprising a frequency-synthesis circuit for generating a stepped-frequency signal having a frequency which can be varied in steps; and a synchronization circuit for synchronizing a tuning oscillator with the stepped-frequency signal, characterized in that the synchronization circuit is arranged to provide an integer frequency-relationship between the stepped-frequency signal and the tuning oscillator.
This object is further achieved in a receiver comprising a tuning system as described above.
This object is further achieved in a method of tuning comprising the steps of generating a stepped-frequency signal having a frequency which can be varied in steps; and synchronizing a tuning oscillator with the stepped-frequency signal; characterized in that the method of tuning comprises the step of providing an integer frequency-relationship between the stepped-frequency signal and the tuning oscillator.
The invention takes the following aspects into consideration. A receiver""s performance, in terms of interference-immunity depends on the spectral purity of its tuning oscillator. This has the following reason. In practice, any receiver comprises at least one mixer circuit which receives a signal from the tuning oscillator. If the tuning oscillator signal comprises a spectral component which does not have the desired oscillator frequency, the mixer circuit will produce spurious mixing products which may cause interference. The interference may manifest itself as, for example, audible whistles in the case of analog AM and FM radio-reception, visual disturbances in the case of analog TV reception, or an increase in bit-error rate of the recovered information in the case of digital transmission.
The tuning oscillator""s spectral purity may be adversely affected if a signal which does not stem from the tuning oscillator itself, leaks into the tuning oscillator. Such signal leakage may be due to, for example, capacitive or inductive coupling between the tuning oscillator and other circuitry in the receiver. The higher the frequency of the leaking signal, the stronger the coupling will be and, consequently, the greater the extent to which the spectral purity of the tuning oscillator will be affected. Furthermore, the nearer in frequency the leaking signal is with respect to the tuning oscillator frequency, the greater the extent to which the spectral purity of the tuning oscillator will be affected. In this respect, it should be noted that, in practice, a signal comprises various frequency components, such as, for example, a fundamental frequency component and harmonic frequency components.
In the background art, the stepped-frequency signal, which is provided by the first PLL, has a frequency of 280 MHz and, consequently, a 1400 MHz fifth harmonic. In practice, this 1400 MHz fifth harmonic will leak into the L-band VCO which has a 1388 MHz oscillation frequency. As a result, the L-band VCO will be parasitically modulated with a 12 MHz frequency which is the difference between the 1388 MHz oscillation frequency and the 1400 MHz fifth harmonic. The L-band VCO""s signal will therefore comprise a 1376 MHz frequency component and a 1400 MHz frequency component. If the L-band VCO drives a mixer circuit for converting input signals in frequency, an input signal whose frequency differs 12 MHz from that of the desired signal will be converted to the same frequency as the desired signal and, consequently, will cause interference.
In accordance with the invention, an integer frequency-relationship between the tuning oscillator and the stepped-frequency signal is provided. If the stepped-frequency signal or any of its harmonics leaks into the tuning oscillator, this will not result in any parasitic modulation because of the integer-frequency relationship. In contradistinction to the background art, such signal leakage will, thus not adversely affect the tuning oscillator""s spectral purity. Consequently, the invention allows a higher spectral purity of the tuning oscillator and, thus provides a receiver which has a better performance in terms of interference-immunity.